The generation and especially the amplification of ultra-short laser pulses with pulse durations smaller than 10 ps is limited by nonlinear effects and damage of optical components. For amplification of ultra-short laser pulses in rare-earth-doped fibers the temporal Kerr-nonlinearity, known as self-phase modulation (SPM), is the first-order and thus the dominating effect. It leads to a pulse quality degradation with increasing pulse energy. Additionally, above a critical peak power, the spatial Kerr-effect, known as self-focussing, leads to dramatic fiber damages.
A well-known approach to overcome these limitations is the chirped-pulse amplification (CPA) technique, whereby optical pulses are first temporally stretched to reduce the pulse peak power during amplification, then amplified by one or more amplification stages and finally recompressed to ultra-short pulse durations. Dispersive elements stretching the pulse duration are for example prisms, gratings, fiber Bragg gratings (FBG), volume Bragg gratings (VBG) or chirped mirrors.
Despite the given advantages, the CPA technique has some great disadvantages. These are alignment sensitivity, non-stability in the long-term-application and high costs due to the additional dispersive elements. Also, the plurality of gratings spoils the compactness of the laser system.
To overcome these disadvantages some developments have been made in the prior art.
Fermann et al. describe in their article “Self-Similar Propagation and Amplification of Parabolic Pulses in Optical Fibers” (Phys. Rev. Lett. 84, 6010-6013 (2000)) a direct amplification of ultra short laser pulses. The effects of dispersion, gain and nonlinearity (self-phase-modulation) are adapted and combined in such a way that a linearly chirped pulse with parabolic spectral and temporal intensity profile arises. However, this approach is limited by the gain bandwidth of the rare-earth doped fiber amplifier and finally by self-focussing. Pulse energies appear to be limited to about 1 μJ.
Another approach is the use of chirped fiber Bragg gratings (CFBG) as dispersive elements in fiber CPA systems, as described in U.S. Pat. No. 5,847,863. Such CFBGs reduce size and complexity of the system. However, due to excessive nonlinearity even at low energies they fail as compressor element. Thus, additional bulky compressor gratings are still needed to extract high pulse energies.
US 2006/0221449 A1 describes the use of CVBGs both as stretcher and compressor. Here, due to their large aperture, these elements are suitable for the compression of energetic pulses to ultra short duration. However, the static nature of their dispersion (fixed by design and fabrication) prohibits an adjustment to the best compression. So additional tuneable dispersive elements need to be implemented.